A turbocharger or a mechanical supercharger (compressor) includes a compression device which is directly or hydraulically driven by a crankshaft (in the case of a compressor) or by an exhaust gas turbine (in the case of a turbocharger). As a rule, turbocharged diesel engines have a large volume between the compression side of the turbocharger and the exhaust of the internal combustion engine. This volume is composed of the volume of the charge air cooler and the volumes of the piping. In contrast, the manifold lengths and the volume between the exhaust of the internal combustion engine and the exhaust gas turbine of the turbocharger is very small.
In the event of a rapid load drop, e.g., during rapid gas ease-off from the full-load range, the pressure upstream from the exhaust gas turbine drops much more quickly than the pressure downstream from the compression device. This results in an unfavorable pressure gradient which may result in fluctuations of the charge pressure or in a recirculation of the compressed fresh air through the compression device (against the nominal flow direction). This causes undesirable, unpleasant flow noises, and increased component stress occurs, which may result in the breakdown of the hydrodynamic lubrication of the turbocharger shaft, for example. This behavior is particularly pronounced in vehicles having a torque converter when the converter bridging clutch is disengaged since, in the event of a load drop, the engine speed and thus also the air mass flow through the engine quickly drop.
The following measures for avoiding this unfavorable pressure situation are known:                closing the throttle valve and opening the exhaust gas recirculation, thereby enabling a reduction of the pressure gradient via the exhaust gas recirculation valve from the intake side to the exhaust side;        raising the engine speed and adjusting via the idling regulation, thereby preventing a rapid drop in the engine speed and increasing the air mass flow through the engine; and        delayed torque reduction via mobility filters, thereby increasing the enthalpy and thus also the pressure between the compression side of the turbocharger and the exhaust of the internal combustion engine as well as the air mass flow through the engine.        
As a rule, the rail pressure is set as a function of the load in common rail systems in such a way that at high loads high rail pressures are set and at low loads low rail pressures are set, for acoustic reasons, among other things. In common rail systems having single-actuator rail pressure regulation (demand-regulated), a reduction in the rail pressure is only possible via injections or leakage losses of the injectors. This results, e.g., in the case of injectors having low leakage losses, in that the high rail pressure cannot be reduced after a rapid load drop. In the event of a subsequent small torque request, e.g., by the idling controller, a loud combustion noise occurs. It is known to reduce the rail pressure using delayed torque reduction via a mobility filter, which results in injections occurring for a longer period of time, which reduces the rail pressure. The same effect may be brought about by limiting the gradient of the driver demand.
The above-described measures for avoiding a critical pressure gradient between the pressure downstream from the compression device of the turbocharger and the pressure upstream from the exhaust gas turbine have a direct effect on the driving performance and thus on the driving comfort. The same is also true for the above-described measures for reducing the rail pressure.
Therefore, it is an object of the present invention to provide a method for avoiding a charge recirculation in the event of rapid load drops in which an effect on the driving performance is reduced as much as possible, as well as reducing the overshooting of the rail pressure during rapid load drops.